The present invention generally relates to asymmetric digital subscriber line control systems, and more particularly to an asymmetric digital subscriber line control system which effectively contains on the side of a feeder line an asymmetric digital subscriber line which makes a two-way transmission of an audio signal and a one-way transmission of an image signal to a subscriber.
Interactive television systems such as video-on-demand, network game, and home shopping systems have been proposed which use a switching network. For example, there is a proposed system which uses the existing telephone line, to make a two-way transmission of the audio signal at 3.4 kHz similarly as in the case of the normal telephone, and to transmit the image signal at 1.5 Mbps from an exchange to a subscriber and to transmit a control signal which requests the desired image signal at 16 kbps from the subscriber to the exchange. Such a subscriber line is called an asymmetric digital subscriber line (hereinafter simply referred to as an ADSL), and an economical configuration of such an ADSL is desirable.
FIG. 1 shows an example of a conventional system which uses the ADSL. The system shown in FIG. 1 includes a subscriber terminal 41, an exchange 42, and a remote station 43. The subscriber terminal 41 includes a telephone set 44, a monitoring television 45, an input operation unit 46, and a circuit terminating equipment 47. The exchange 42 includes a circuit terminating equipment 48 and a talk switch 49. An image providing unit 50 is formed by an image providing enterprise that provides movie programs and various kinds of image information, and sends a selected movie program or image information depending on a request from the subscriber.
In a case where the distance from the exchange 42 to the subscriber terminal 41 is short, no remote station 43 is provided, and the exchange 42 is connected directly to the subscriber terminal 41 via a line. On the other hand, in a case where the distance from the exchange 42 to the subscriber terminal 41 is long, the remote station 43 is provided as shown in FIG. 1. In actual practice, a plurality of subscriber terminals 41 are connected to this remote station 43.
A line DPL between the remote station 43 and each subscriber terminal 41 is referred to as a drop line, and a line FDL between the remote station 43 and the exchange 42 is referred to as a feeder line. The drop line DPL is formed by the existing telephone line which is made up of a twisted-wire cable, a coaxial cable, or a combination thereof. On the other hand, there are proposals to form the feeder line FDL by an optical fiber cable.
The input operation unit 46 of the subscriber terminal 41 is made up of a ten-key, a keyboard or the like, The circuit terminating equipment 47 is connected to the drop line DPL. The telephone set 44 can talk with an arbitrary subscriber. The monitoring television 45 receives the image signal from the image providing unit 50 and displays an image described by the image signal. The circuit terminating equipment 47 carries out the multiplexing and demultiplexing of the audio signal, the demultiplexing of the image signal, and multiplexing of the control signal.
On the other hand, the circuit terminating equipment 48 of the exchange 42 carries out the multiplexing and demultiplexing of the audio signal, the demultiplexing of a control signal CNTS from the subscriber terminal 41, and multiplexing of an image signal VDS to be transmitted to the subscriber terminal 41. If the feeder line FDL is formed by the optical fiber cable, the circuit terminating equipment 48 is provided with the functions of converting both optical and electrical signals. For example, a desired movie program out of the plurality of movie programs provided by the image providing unit 50 is selected based on the demultiplexed control signal CNTS.
For example, the image signal VDS may be coded by MPEG1 which is one of the color motion picture coding techniques which are proposed for standardization (International Standard IS11172 set by ISO in November 1992), so as to transmit the image signal VDS at a rate of 1.5 Mbps. The coded image signal may be transmitted using the existing telephone line by utilizing a 64-quadrature amplitude modulation (64-QAM). In addition, the control signal may be transmitted at a rate of 16 kbps, for example.
In the drop line DPL, the audio signal is transmitted in two directions, the image signal is transmitted in one direction towards the subscriber terminal 41, and the control signal is transmitted in one direction towards the exchange 42. In other words, not all transmissions via the drop line DPL are made in two directions, and some signals are transmitted in only one direction. This is the reason why such a drop line DPL is referred to as the ADSL.
On the other hand, in the feeder line FDL between the exchange 42 and the remote station 43, 3 lines made up of an audio signal line for transmitting the audio signal, an image signal line for transmitting the image signal and a control signal line for transmitting the control signal are allocated for each subscriber terminal 41 so as to transmit the multiplexed optical signal.
Therefore, at the exchange 42 and the remote station 43, it was necessary to allocate and process 3 lines associated with each subscriber terminal 41, that is, the audio signal line, the image signal line and the control signal line. For this reason, when the number of subscriber terminals 41 to be contained increases, the number of lines to be allocated increases considerably, and there was a problem in that it becomes necessary to expand the installations of the exchange 42 and the remote station 43. There was also a problem in that the system becomes expensive when such expansion of the installations is made.